Non-aqueous secondary batteries, typified by lithium ion secondary batteries, have been widely used as main power sources for portable electronic devices such as cellular phones or notebook computers. Further, application of non-aqueous secondary batteries has been expanded to main power sources for electric automobiles or hybrid cars, and to systems for storing nighttime electricity. Along with the popularization of non-aqueous secondary batteries, securing stable battery performance and safety has become a challenge.
Separators play an important role in securing safety for a non-aqueous secondary battery. Particularly from the viewpoint of a shutdown function, polyolefin porous membranes having polyolefin as a main component are currently used.
However, a separator which is composed only of a polyolefin porous membrane may be melted in its entirety (a so-called meltdown) when the separator is exposed to a temperature higher than the temperature at which a shutdown function occurs. Since polyolefin is poorly adhesive to other resins or other materials, adhesion between a polyolefin porous membrane and an electrode is not sufficient, which has caused deterioration in battery capacity or deterioration in cycle characteristics in some cases.
Accordingly, a suggestion has been made to provide a porous layer containing a resin and a filler on one side or both sides of a polyolefin porous membrane for the purpose of improving the heat resistance of a separator or improving the adhesion between an electrode and a separator (see, for example, Patent Documents 1 to 9).
Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 2000-030686
Patent Document 2: Japanese National-Phase Publication (JP-A) No. 2012-529742
Patent Document 3: JP-A No. 2011-171290
Patent Document 4: JP-A No. 2010-065088
Patent Document 5: JP-A No. 2000-057846
Patent Document 6: JP-A No. 2009-021265
Patent Document 7: JP-A No. 2006-286531
Patent Document 8: JP-A No. 2009-187702
Patent Document 9: JP-A No. 2002-141042